This invention relates to a scrubbing process and in particular to a process for scrubbing chlorine from a gaseous stream containing chlorine. An example of such a chlorine-containing gas stream is the vent gas from a plant producing chlorine: the vent gas often comprises oxygen, carbon dioxide and chlorine remaining as a result of incomplete condensation of the chlorine produced in the chlorine stream resulting from the electrolysis of brine. Other sources of chlorine-containing gas include reactor bleeds and off-gases resulting from the oxidation of chlorinated hydrocarbons; blow-down during chlorine tanker off-loading; pressure relief streams from chlorine processing plants; and primary brine dechlorination through air sparging. For environmental reasons it is desirable to remove essentially all of the chlorine from the gas stream before the latter is discharged to the atmosphere.
The chlorine removal is conventionally effected by scrubbing with an alkaline solution, often caustic soda solution, thereby forming a solution containing hypochlorite ions by the reaction
Cl2+2OHxe2x88x92xe2x86x92Clxe2x88x92+OClxe2x88x92+H2O
Thus one hypochlorite ion is formed for each molecule of chlorine scrubbed from the gas. Generally the resulting hypochlorite-containing liquor is treated to decompose the hypochlorite ions into oxygen gas and chloride ions before discharge of the liquor, i.e. according to the reaction
2OClxe2x88x92xe2x86x922Clxe2x88x92+O2xe2x88x92
It has been proposed in U.S. Pat. No. 4,297,333 to effect such decomposition of hypochlorite ions in a liquor obtained by the caustic scrubbing of chlorine from a tail gas from an electrolytic chlorine plant by passage of the hypochlorite-containing liquor through a bed of a nickel oxide containing catalyst.
The scrubbing operation is usually operated in a cyclic process: thus the chlorine-containing gas is passed through a scrubber, e.g. a contactor, of which a packed tower is the most common, through which is flowing an aqueous scrubbing liquor generally counter-current to the flow of gas. The scrubbing liquor scrubs the chlorine from the gas giving an effluent liquor laden with chloride and hypochlorite ions. This effluent liquor leaving the scrubber, e.g. tower, is then recycled, usually via a holding tank, pump, and one or more heat exchangers, e.g. to the top of the tower.
The process may be operated as a continuous process, in which case, fresh alkali solution is added, continuously or periodically, before recycle, and part of the circulating liquor is removed, continuously or periodically, as a purge stream. This purge stream, a hypochlorite solution generally containing some excess of alkali, may be recycled, after treatment, for example as aforesaid, to decompose the hypochlorite therein, e.g., to a chlorine production plant. Alternatively the purge may be taken as a hypochlorite product stream or discharged to drain, generally after destruction of the hypochlorite therein.
Alternatively the process may be operated as a batch or semi-continuous process. In such processes, a suitable reservoir, e.g. a holding tank, is provided in the circulation loop and initially this reservoir is charged with fresh alkali. The scrubbing operation is operated until the alkali concentration drops to a predetermined level and then some or all of the liquor in the circulation loop is discharged and replenished with fresh alkali. In some cases the system may be designed such that the scrubbing operation may be halted or switched to another scrubber during such discharge and replenishment of the liquor.
Usually the circulation rate, rate of addition (if any) of fresh alkali, and amount of purge (if any) are such that the circulating liquor has a maximum sodium hypochlorite ion content of the order of 12-15% by weight, although it may be lower as a result of natural decomposition of the hypochlorite or the use of more dilute alkali solutions. Lower hypochlorite concentrations may result where other acid gases such as carbon dioxide, sulphur oxides, and hydrogen chloride are also present in the gas stream and these are co-absorbed with the chlorine.
Attempts to operate with higher hypochlorite concentrations tend to result in the decomposition of hypochlorite with the formation of chlorates
3NaOClxe2x86x922NaCl+NaClO3xe2x80x83xe2x80x83Reaction 1
The formation of chlorates is generally undesirable since they tend to be explosive and very toxic. Furthermore the rate of reaction 1 is strongly affected by the pH and temperature as well as the hypochlorite concentration. When the alkali is exhausted, i.e. the system is over chlorinated, the rate of chlorate formation is greatly accelerated. Under these conditions hypochlorous acid may be formed by the reaction
NaOCl+Cl2+H2Oxe2x86x922HOCl+NaClxe2x80x83xe2x80x83Reaction 2
The following reactions may then also occur
2HOCl+NaOClxe2x86x92NaClO3+2HClxe2x80x83xe2x80x83Reaction 3
HCl+NaOClxe2x86x92HOCl+NaClxe2x80x83xe2x80x83Reaction 4
The excess of chlorine thus favours reaction 2 and hence reaction 3. The rate of reaction 3 is much greater than that of reaction 1 and is strongly exothermic. The reaction thus has a runaway potential. This is normally avoided by providing for an excess of alkali and by cooling and/or by operating at a lower hypochlorite concentration.
In order to minimise the risk of chlorate formation while at the same time enable the system to cater for plant upsets producing an increased amount of chlorine in the gas being treated, it is desirable to operate at a lower circulating hypochlorite content. While this can be achieved by increasing the size of the holding tank and providing for significant increase in the circulation rate and amount of caustic soda added when such increases in the chlorine content occur, such modifications are not attractive economically. It is generally desirable to operate with a sufficient excess of caustic soda that increases in the chlorine content of the feed gas can be accommodated without providing for control of the amount of added caustic soda or of the circulation rate. In the case where changes in chlorine feed rate are due to emergency relief, these changes normally arise too fast for a control system to respond.
In the present invention this problem is overcome by providing for the catalytic decomposition of hypochlorite ions in at least part of the circulating liquor before it is recycled to the scrubber.
Accordingly the present invention provides a process for the scrubbing of chlorine from a chlorine-containing gas comprising contacting said gas with an aqueous feed liquor containing an excess of alkali over that required to react with the chlorine in the gas whereby said chlorine is scrubbed from the gas to provide an effluent liquor containing chloride and hypochlorite ions resulting from the reaction of said chlorine gas with said alkali, and passing at least part of said effluent liquor through a fixed bed of a catalyst for the decomposition of hypochlorite ions whereby hypochlorite ions in said at least part of the effluent liquor are decomposed to oxygen gas and chloride ions to give a treated liquor containing a decreased concentration of hypochlorite ions, characterised in that, for at least part of the time while said gas is being contacted with said aqueous feed liquor, at least part of said treated liquor is recycled as at least part of the aqueous feed liquor.
Thus in the invention, some of the hypochlorite produced from the reaction of chlorine with the alkali is decomposed to chloride ions and oxygen. As a result the standing concentration of hypochlorite in the circulating liquor can be decreased with consequent reduction in the risk of runaway reactions and chlorate formation. Further, as a result of the decreased risk of runaway reactions, it is possible to operate at higher temperatures, thereby reducing the need for cooling of the circulating liquor. Operation at higher temperatures is also beneficial as the rate of decomposition of hypochlorite is increased and so the volume of catalyst required may be relatively small.
Where the process is operated in a batch, or semi-continuous, mode, i.e. where the circulating liquor is periodically discharged and replenished, it may be desirable to effect the discharge by purging all the liquor through the catalyst bed. In this way the discharged liquor has passed through the bed and a substantial proportion of the hypochlorite in the liquor may thus be decomposed prior to disposal.
Where the process is operated continuously with a continuous or periodic purge, it is preferred that the purge stream is taken from the mixture of the treated liquor and the remainder, if any, of the effluent liquor. Conveniently the treated liquor and the remainder of the effluent liquor, if any, are fed to a holding tank or reservoir to which the solution of fresh alkali is added. It is also generally convenient that the purge is taken from the holding tank or from the recycle liquor taken from the holding tank before the latter is fed to the scrubber even though such a purge will inevitably contain some of the fresh alkali. Where the purge stream is recycled to a chlorine production plant, such alkali is not wasted. Where the purge stream is not recycled, but is discharged as a hypochlorite stream or to drain (generally after decomposition of the hypochlorite therein) there may be economic advantages in that the amount of alkali that is discharged in the purge stream may be decreased.
In an alternative process, two scrubbing loops may be employed: while one is on scrubbing duty, spent liquor is discharged from the other loop and then the latter is replenished. Thus the gas is fed alternately to the scrubber of the first loop, during which time aqueous liquor is discharged from the reservoir of the second loop and the reservoir of the second loop is replenished with fresh aqueous alkali solution, and to the scrubber of the second loop, during which time aqueous liquor is discharged from the reservoir of the first loop and the reservoir of the first loop is replenished with fresh aqueous alkali solution; and, for at least part of the period when the gas is being fed to the scrubber of the first loop and for at least part of the period when the gas is being fed to the scrubber of the second loop, at least part of the effluent liquor is passed through the catalyst bed, before or after passage through the reservoir of the respective loop, to give the treated liquor which is mixed with the remainder, if any, of the effluent liquor, and recycled as the aqueous feed liquor fed to the scrubber of the respective loop.
Instead of employing two separate scrubber units, each having its own scrubber and reservoir, a single scrubber unit having two reservoirs may be employed: while one reservoir is supplying the aqueous feed liquor, spent liquor is discharged from the other reservoir and then the latter is replenished. Thus the effluent liquor is recycled alternately via a first reservoir during which time aqueous liquor is discharged from a second reservoir which is then replenished with fresh aqueous alkali solution and via the second reservoir during which time aqueous liquor is discharged from the first reservoir which is then replenished with fresh aqueous alkali solution; and, for at least part of the period when the effluent liquor is recycled from the first reservoir and for at least part of the time when the effluent liquor is recycled from the second reservoir, at least part of the effluent liquor is passed through the catalyst bed, before or after passage through the respective reservoir, to give the treated liquor which is mixed with the remainder, if any, of the effluent liquor, and recycled as the aqueous feed liquor.
In the present invention some or all of the effluent liquor is passed through a fixed bed containing a hypochlorite decomposition catalyst. Unless it is desired that the hypochlorite content of the recycled aqueous liquor fed to the scrubber is maintained at a low level, e.g. 500-5000 ppm by weight, often it is only necessary for a relatively small proportion, for example 2 to 50%, of the effluent liquor to be fed to the catalyst bed. The destruction of hypochlorite ions in a catalyst bed is described in, inter alia, U.S. Pat. No. 5,387,349. In that reference it is indicated that it is necessary to employ a number of catalyst beds in series: however since in the present invention it is not necessary to obtain a catalyst bed effluent liquor having a very low level of hypochlorite ions, it is not necessary to avoid back mixing. As a consequence the apparatus employed may be simpler than that of that reference and a series of catalyst beds may not be needed.
The liquor being treated by the catalyst may be passed upwards through the catalyst bed, i.e. xe2x80x9cupflowxe2x80x9d where the liquor flow is co-current with the flow of evolved gas, or down through the catalyst bed, i.e. xe2x80x9cdownflowxe2x80x9d where the liquor flow is counter-current to the flow of evolved gas.
The aforesaid U.S. Pat. No. 5,387,349 indicated that it was desirable to operate a downflow system under conditions such that the rate of disengagement of the oxygen produced by the decomposition of hypochlorite is less than 0.05 m3 per second per m2 of gas disengagement surface, i.e. cross sectional area of the catalyst bed. By using an upflow system, it is possible to operate the present invention at higher space velocities than was contemplated by that reference and so relatively small catalyst volumes may be employed. The volume of catalyst is usually such that the liquid hourly space velocity through the catalyst is in the range 0.1 to 1000 hxe2x88x921. An upflow system is desirable where the liquid flow velocity and/or the rate of evolution of gas is relatively large.
Since, in many cases, only a small proportion, e.g. 2 to 50%, of the effluent liquor need be passed through the catalyst, and high space velocities employed, the volume of catalyst can be relatively small.
For example for a plant having an effluent liquor rate of about 100 m3/h, if only 5% of the effluent liquor is passed through the catalyst, the flow rate of liquor passing through the catalyst is 5 m3/h. If the space velocity is 50 hxe2x88x921, the volume of catalyst required is only 100 litres. If the catalyst is disposed as a bed of depth 20 cm, the area of the bed cross section, i.e. the area available for gas disengagement is about 0.5 m2. If a downflow arrangement is employed and problems of the gas evolution disrupting the liquor flow through the catalyst are to be avoided, the rate of gas evolution should not exceed 0.025 m3 per second. This rate of gas evolution is given by the decomposition of about 600 kg/h of sodium hypochlorite and so the scrubber effluent liquor should contain no more than about 12% by weight of sodium hypochlorite. If the effluent liquor has a lower hypochlorite content, the space velocity could be increased, thereby enabling a smaller amount of catalyst to be used.
As indicated above, it is not normally necessary to employ conditions such that the treated liquor has a very low hypochlorite content: typically the treated liquor will have a hypochlorite content in the range 100 to 10000 ppm, i.e. about 0.1 to 10 g/l. Generally it is sufficient for the catalyst to effect decomposition of at least 80%, particularly 90 to 99%, of the hypochlorite in the liquor stream fed to the catalyst bed. The temperature of the liquor fed to the catalyst is typically in the range 10 to 90xc2x0 C., particularly 20 to 60xc2x0 C., and especially 30 to 50xc2x0 C.
In another embodiment, the concentration of hypochlorite in the circulating liquor is maintained at a low level by passing a major proportion, or all, of the effluent liquor through the bed of hypochlorite decomposition catalyst.
The catalytic decomposition of hypochlorite ions by a fixed bed of catalyst is well known. Suitable catalysts include metal oxides such as nickel and/or cobalt oxide, often promoted with oxides of other metals such as copper, aluminium, iron, and magnesium, supported on, or bound together by a suitable binder into, shaped particles. Suitable support and binder materials capable of withstanding the alkaline conditions include certain plastics materials such as polyvinylidene fluoride, and oxidic materials such as magnesia, alpha-alumina and calcium aluminate cement. Particularly suitable catalysts are those described in EP 0 211 530, EP 0 276 044, EP 0 397 342 and WO 97/04870.